1. Field of the Invention
The present invention relates generally to sensing apparatus used to detect the presence of granular material at a predetermined location in conduit adapted to contain the flow of granular material and in containers, such as hoppers, used to store granular material. More particularly, the invention relates to an electrical sensing terminal in such apparatus that is shielded by a protective housing to reduce the damaging effects of flowing and impinging granular material on the terminal.
2. Description of the Prior Art
Granular materials such as corn, barley and powdered metals and glasses are often stored in containers such as bins and hoppers that are usually equipped with a gate or valve actuated discharge throat. The granular material is frequently transported to and from the container through conduit using various means for propulsion such as pneumatic power, gravity feed and even pumping if the material is in the form of a slurry. Various sensing apparatus are used to indicate the level of granular material in the containers for inventory purposes, to locate "plugged material", and to control material input and discharge.
Heretofore, many sensing apparatus have been constructed that detect the presence of granular material by means of a "sensor" placed at a predetermined location in containers and conduit. As used herein the term "sensor" indicates that element of a sensing apparatus which is usually located in the proximity of the aforementioned predetermined location and contains a "sensing terminal" that is adapted to exhibit a response to communication with granular material. The response of the sensing terminal is generally monitored by means of known electrical circuitry. Five sensors widely used in sensing apparatus for granular materials may be generally classified as diaphragm, paddle, frequency vibration, ultrasonic and antenna types.
Diaphragm-type sensors are usually quite large and may measure as much as 8 inches on a side. Thus, they have the tendency to restrict material flow especially if mounted in a small diameter discharge throat. This type of sensor is also difficult to mount and requires bolting or welding to the container wall and must be frequently replaced when used in conjunction with abrasive or corrosive granular materials.
Paddle-type sensors have sensing terminals in the form of paddles or vanes that are adapted to stall when in contact with granular material. Paddle-type sensors may extend up to 6 inches into the container or conduit so they tend to restrict material flow when mounted in a narrow aperture such as a discharge throat and to wear rather quickly when used in conjunction with abrasive materials. Another disadvantage is that a large space on the exterior of the container is required to provide clearance for a paddle drive motor and associated electrical circuitry.
Frequency vibration- and antenna-type sensors have the drawback that they require sensitive electronic equipment that often breaks down when used in dusty, moisture laden or corrosive atmospheres. Also, a large space on the exterior of the container is needed to provide clearance for the electronic equipment.
Ultrasonic sensors generally employ sensing terminals in the form of two aligned ultrasonic transducers separated by a predetermined distance. Because it is critically important that the transducers remain aligned during operation, this type of sensor requires frequent adjustment. The delicate transducers do not readily withstand the abrasion caused by the flow of many granular products and may require a support arm which extends up to 10 inches into the container or conduit. Thus, they are also not readily adaptable for use in areas of restricted diameter such as discharge throats.